1. Field of the Invention
This invention relates to a phase control system for a video tape recorder using a helical scanning system which is capable of carrying out variable speed reproduction.
2. Description of Prior Art
In the video tape recorder, a demand has been raised for a function to reproduce signals recorded on the magnetic tape at speeds different from the normal speed (to be hereinafter called a special reproduction).
In the case of such special reproduction, however, since the path of the rotary video head travelling on the magnetic tape is different from the recorded path, one recorded path cannot accurately be scanned so as to generate a time period during which the reproduced output signal is reduced.
In such particular reproduction speed modes as the reversed normal speed motion mode, the still mode, the double speed fast motion mode, and the triple speed fast motion mode, the time period during which the reproduction output signal is reduced can be positioned in the vicinity of the vertical synchronizing signal, thereby enabling a television receiver to obtain a proper reproducing picture substantially free from the generation of noise bars.
It is necessary for performing the special reproduction to transport the magnetic tape accurately under phase control so that the path of the rotary video head is controlled so as to position the time period during which the reproduction output signal is reduced in the vicinity of the vertical synchronizing signal. Also, in the normal reproduction mode, the magnetic tape has to be transported under accurate phase control so that the rotary video head travelling on the magnetic tape accurately scans the recorded path. For an accurate phase control, a method has hitherto been used which records a control signal at every frame, during the recording, on a control track provided at the magnetic tape lengthwise thereof and reproduces the recorded control signal during the reproduction, so that the tape is subjected to phase control by the use of the reproduced control signal. In this case, unless a distance (to be hereinafter called a value X) between the positions of recorded video track and control signal on the magnetic tape, when recorded by a video tape recorder of the same standard, is determined to be a constant value, it is impossible for the phase control on the tape transport to allow the rotary video head to accurately scan the recorded video track during the reproduction.
However, in fact, an error is caused by the mechanical accuracy on the mounting position of the control head for recording the control signal, so that variations in the value X are produced even in the video tape recorder of the same standard. Hence, in some cases, the rotary video head cannot scan the recorded video track accurately.
Conventionally, an adjustment of the reference phase for phase comparison with the phase control system has corrected the variations in the value X, which has been useful only for the normal reproduction mode. Since the correction values are different in the special reproduction modes, even when the variations in value X are corrected for the normal reproduction mode, there has been a defect in such that the value X should be corrected independently for each special reproduction mode.